Water is a molecule, which is the fundamental unit of a chemical compound formed when two or more atoms join together. The chemical formula H2O confirms this structure, indicating that two hydrogen atoms are chemically bonded to a single oxygen atom. This simple, three-atom arrangement is responsible for the unique and life-sustaining properties of water across the planet. The importance of water as a solvent and a medium for biological reactions stems from how these atoms connect and interact.
Defining the Water Molecule
The internal structure of a single water molecule is held together by a strong type of connection known as a covalent bond. This bond forms when the oxygen atom and each of the two hydrogen atoms share electrons between them. The sharing of an electron pair between the non-metal atoms creates a stable link, allowing the atoms to achieve a full outer electron shell configuration. Specifically, the oxygen atom forms two separate covalent bonds, one with each hydrogen atom, resulting in the stable H2O unit.
The Unique Shape and Polarity
While the atoms within water share electrons, this sharing is not equal, leading to a specific charge distribution across the molecule. Oxygen is a more electronegative atom, meaning it has a stronger pull on the shared electrons than the hydrogen atoms do. Consequently, the shared electrons spend more time orbiting the oxygen nucleus, giving the oxygen side a partial negative charge. This unequal distribution leaves the two hydrogen atoms with a partial positive charge.
The molecule also adopts a distinctive “bent” geometry, with the two hydrogen atoms positioned at an angle of approximately 104.5 degrees from one another. This shape is caused by the repulsive force of the two pairs of unshared electrons located on the oxygen atom. Because the charges are separated and the molecule is not linear, water is considered a polar molecule, possessing a distinct positive end and a negative end.
The Power of the Hydrogen Bond
The polarity of individual water molecules causes them to be attracted to one another through an intermolecular force called a hydrogen bond. This attraction forms when the partial positive hydrogen end of one water molecule seeks out and links to the partial negative oxygen end of a neighboring molecule. Although a single hydrogen bond is much weaker than the covalent bonds holding the molecule together, the sheer number of these attractions creates a cohesive network in liquid water.
This extensive network of hydrogen bonds is directly responsible for many of water’s unusual properties:
- The bonds require a significant amount of energy to break, giving water a high specific heat capacity, which allows it to absorb and release heat slowly and help stabilize temperatures.
- The attraction between molecules also creates surface tension, allowing insects to walk on water.
- Hydrogen bonding enables capillary action, which is how plants draw water up their stems.
- As water freezes, hydrogen bonds lock the molecules into a highly ordered, open crystalline lattice, which is why solid ice is less dense than liquid water and floats.
- Water’s polarity and hydrogen bonding capability make it an excellent solvent, as it can surround and dissolve many other polar or charged substances.